This invention relates to a method of modifying rubber with a halogen or halogens; specifically to a process for the production of halogenated, ethylenically unsaturated rubber. More specifically, it is directed to a continuous process for the manufacture of halogenated elastomers such as butyl (a copolymer of a major proportion of an isoolefin and a minor proportion of a multiolefin), EPDM (a terpolymer of ethylene, propylene and a nonconjugated diene), SBR (styrene-butadiene rubber), BR (polybutadiene rubber), polyisoprene rubber, etc.
Numerous references teach methods for halogenating various polymers, and more particularly ethylenically unsaturated elastomers. There are, for example, references that teach the halogenation of butyl rubber, but each suffers from serious limitations. An early reference, U.S. Pat. No. 2,944,578, teaches that chlorinated butyl rubber can be produced in a batch process by dissolving butyl rubber in a suitable, nonreactive solvent, e.g., hexane, and introducing chlorine or a chlorinating agent. By suitable control of the temperature, concentrations of chlorinating agent and rubber, and reaction time, chlorinated rubber containing the desired level of chlorine is produced. However, a batch process is inherently inefficient and the need to dissolve the rubber in a solvent incurs significant expenses for solvent recovery and environmental control.
An improved, continuous process for chlorination or bromination of butyl rubber was subsequently disclosed in U.S. Pat. No. 3,099,644. However, that process still required the preparation and use of a solution of butyl rubber, which, in addition to the limitations noted above, is limited as to the concentration of rubber which can be processed, and which requires significant equipment and process control to precipitate the halogenated rubber from solution and then dry the rubber in a controlled manner so as to avoid degradation. The halogenation of ethylene-propylene nonconjugated diene elastomers (EPDM) has also been disclosed; such processes are analogous to those for halogenating butyl rubber. For example, U.S. Pat. No. 4,051,083 describes the solution bromination and chlorination of EPDM using N-halosuccinimide; additionally, the "neat" halogenation of EPDM is also described. In the latter disclosure the halogenating agent is dispersed in the EPDM by blending on a cool rubber mill and halogenation is effected by heating the mixture in a hydraulic press.
Halogenation of EPDM in an aqueous batch process is disclosed in U.S. Pat. No. 3,896,095. The process employs the addition of an excess of Cl.sub.2 or Br.sub.2 to a polymer slurry to effect halogenation and avoid the expense of solvent recovery systems previously disclosed for solution halogenation processes.
Chlorobromination of polymers such as polybutadiene, butadiene-isoprene copolymers and natural or synthetic polyisoprene is disclosed in British Pat. Nos. 1,483,063 and 1,483,064. The reaction is described as taking place at a low temperature of 0.degree.-15.degree. C., preferably in an inert solvent, and the halogenated products are described as containing high levels, e.g., at least 55% by weight of halogen.
A close reading of these references indicates the difficulty with which halogenation of elastomers has been conducted prior to the invention disclosed herein. The various limitations of these batch and continuous solution processes are overcome by the improved process of the present invention.
The possibility of producing a halogenated rubber such as halogenated butyl rubber continuously in an extruder-reactor has been recognized, see, e.g., U.S. Pat. No. 4,185,057. However, the generalized disclosures of that reference do no more than acknowledge the desirability of such a process, but do not teach one how to accomplish such a process. The reference suggests that only enough chlorine be introduced into the extruder to react with the butyl rubber so that no chlorine remains after reaction. It then goes on to suggest that another gas, e.g. nitrogen be introduced to effect the production of gas filled pores in the finished rubber, which is the primary object of the invention.
No examples are disclosed in the patent and no conditions disclosed which would enable one to actually conduct such a butyl halogenation process. The invention disclosed herein provides a teaching sufficient to enable the practice of this unique halogenation process and apply such a teaching to the halogenation of butyl rubber.
Chlorination of butyl rubber using dichloramine-T and a calender has been reported by Bulgarian workers (Kh. Tenchev, et al, Chem Abstracts 50756u). The disclosed process was not intended to produce neat chlorinated butyl since calendering is carried out on a mixture of butyl rubber, accelerators, prevulcanization inhibitors as well as variable amounts of carbon black and dichloramine-T.
The halogenation, in a kneader or extruder, of polymers other than butyl rubber using reagents that differ from those disclosed herein is described in U.S. Pat. No. 3,364,187. The polymers containing carboxylic acid groups are converted to the acyl halide derivatives using specific halogenating agents. The patent suggests that the kneading step may be carried out in an extruder, a Banbury mixer, a roll mill or any other apparatus that yields the described kneading action.
A British Patent, No. 1,257,016, discloses a process for treating polymers with halogenating agents such as N-bromosuccinimide under mechanical shear for the purpose of producing unsaturation. The patent mentions that halogenation may possibly occur in an intermediate step followed by dehydrohalogenation, but production and isolation of a useful halogenated product is not an objective, nor is it achieved. The process also requires the use of scavenging amounts of a metal oxide or carbonate such as magnesium oxide, zinc oxide or calcium carbonate in addition to the halogenating agent and .alpha.-olefin polymer. The patent discloses, as an alternate method, the preblending of the halogenating agent with a solution of the polymer followed by solvent removal. It is stated that very little, if any, reaction occurs during such an operation.
An extensive disclosure of polymer modifications conducted in an extruder can be found in U.S. Pat. No. 3,862,265. This patent is directed to modification of polyolefins using heat, shear and controlled pressure to induce degradation in the polyolefin and to combine the polyolefin with a free-radical initiator and/or one or more monomers. The broad disclosure is of value for its teachings directed to the modification of polyolefins with various monomers especially to form novel grafted polymers.
The particular sensitivity of butyl rubber when exposed to shear and elevated temperatures in the presence of a halogenating agent has made the achievement of a halogenated butyl product using an extruder-reactor a difficult goal, and until the invention disclosed herein, a goal that had not yet been achieved. The halogenation reaction of butyl rubber in solution is described with "Encyclopedia of Chemical Technology," Kirk-Othmer, Third Edition (1979), Volume 8 at page 476 ff. It is noted that the halogenation reaction carried beyond one halogen atom per olefin unit is complicated by chain fragmentation. Indeed, such fragmentation or degradation is a persistent problem when halogenation of butyl rubber is attempted; that problem is aggravated under conditions of heat and shear.
An additional difficulty is the dehydrohalogenation reaction and stabilizers are normally added to solution halogenated butyl to protect against this reaction during processing. It is also necessary to avoid other side reactions such as halogenation of isobutylene residues. Both of the aforementioned reactions are further aspects of the sensitivity of the polymer to the severe halogenation reaction that has made the achievement of controlled halogenation of neat butyl in an extruder-reactor a previously elusive goal.
Conventional processes, i.e., those which halogenate butyl rubber in solution, incur significant disadvantages. These include high capital investment for the equipment needed to handle, purify, and recycle the solvent, high energy costs for the movement, vaporization, and purification and recycle of the solvent, large hydrocarbon emissions to the atmosphere and the use of considerable space for the equipment necessary to handle large volumes of solutions.